The primary consideration in the design of geometric cross sections for highways are run-ways, and taxiways is drainage. Details vary depending on the type of facility Highway cross sections consist of travelled way, shoulders (or parking lanes), and drainage channels. Shoulders are intended primarily as a safety feature. They provide for accommodation of stopped vehicles, emergency use, and lateral support of the pavement. Shoulders may be either paved or unpaved. Drainage channels may consist of ditches (usually grassed swales) or of paved shoulders with berms or curbs and gutters. Cross section of various roads is given bellow.
Pavement surface characteristics
For a safe and comfortable driving four aspects of the pavement surface are important; the friction between the wheels and the pavement surface, smoothness of the road surface, the light re action characteristics of the top of pavement surface, and drainage to water.
Friction
Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe operating speed. Further, it also acts as the acceleration and deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of vehicles. Skidding happens when the path travelled along the road surface is more than the circumferential movement of the wheels due to friction Slip occurs when the wheel revolves more than the corresponding longitudinal movement along the road. Various factors that act on friction are:
The frictional force that develops between the wheel and the pavement is the load acting multiplied by a factor called the coefficient of friction and denoted as of the choice of the value of friction is a very complicated issue since it depends on many variables. IRC suggests the coefficient of longitudinal friction as 0.35-0.4 depending on the speed and coefficient of later friction as 0.15. The former is useful in sight distance calculation and the latter in horizontal curve design.
Unevenness
It is always desirable to have an even surface, but it is seldom possible to have such one. Even if a road is constructed with high quality pavers, it is possible to develop unevenness due to pavement failures. Unevenness a etc. the vehicle operating cost, speed, riding comfort, safety, fuel consumption and wear and tear of tyres. Unevenness index is a measure of unevenness which is the cumulative measure of vertical undulation of the pavement surface recorded per unit horizontal length of the road. An unevenness index value less than 1500 mm/km is considered as good, a value less than 2500 mm.km is satisfactory up to speed of 100 kmph and values greater than 3200 mm/km is considered as uncomfortable even for 55 kmph.
Light reflection
Drainage
The pavement surface should be absolutely impermeable to prevent seepage of water into the pavement layers. Further, both the geometry and texture of pavement surface should help in draining out the water from the surface in less time.
Camber
Camber or cant is the cross slope provided to raise middle of the road surface in the transverse direction to drain o rain water from road surface. Too steep slope is undesirable for it will erode the surface. Camber is measured in 1 in n or n% (E.g., 1 in 50 or 2%) and the value depends on the type of pavement surface.
Width of carriage way
Width of the carriage way or the width of the pavement depends on the width of the traffic lane and number of lanes. Width of a traffic lane depends on the width of the vehicle and the clearance. Side clearance improves operating speed and safety.
Kerbs
Kerbs indicates the boundary between the carriage way and the shoulder or islands of footpaths. Different types of kerbs are
- Low or mountable kerbs:
- Semi-barrier type kerbs:
- Barrier type kerbs:
Road margins
The portion of the road beyond the carriageway and on the roadway can be generally called road margin. Various elements that form the road margins are given below.
- Shoulders
- Parking lanes
- Bus-bays
- Service roads
- Cycle track
- Footpath
- Guard rails
